This invention relates to a method of encapsulating an active material in a protective coating material immiscible therewith, and the encapsulated active material of this method.
Coating or microencapsulation of active materials is commonly done to protect these materials from the environment, or to control their release or improve their handling. Many coating or microencapsulating techniques have been described in the art. For example, Somerville (U.S. Pat. No. 3,015,128, incorporated herein by reference) describes a centrifugal encapsulating apparatus that produces mass quantities of relatively small individual capsules of solid or liquid active material which may have substantially uniform and predetermined characteristics. In a later patent (U.S. Pat. No. 3,310,612, incorporated herein by reference), Somerville describes a method and apparatus for centrifugally forming high quality capsules having a diameter as large as 1500 microns.
Johnson et al. (J. of Gas Chromatography, 345, (1965), incorporated herein by reference) describes a method for coating glass beads with a mixture of liquid phases and particles of diatomaceous earth which is reproducible and improves column efficiency.
Harlowe ("Scale-Up Problems Associated with Microencapsulated Systems" The Arden House Conference, pp 1-2, Feb. 13-18, 1983, incorporated herein by reference) describes both a submerged nozzle device, which can be used to produce capsules between 1200 and 2500 microns, and a centrifugal extrusion apparatus, which can produce capsules in the 500- to 1000-micron range.
Anderson et al. (U.S. Pat. No. 4,764,317, incorporated herein by reference) addresses the problem of capsule breakage, due in large measure to collision of capsules, by providing a continuous collection system for microcapsules with a liquid fill which protects the capsules and reduces capsule breakage.
Sparks et al. (U.S. Pat. No. 4,675,140, incorporated herein by reference) describes a method and apparatus for coating or microencapsulating both solid particles and viscous liquid droplets which enables at least a majority of the particles to be coated individually or discretely rather than in clusters while simultaneously providing improved means for separating unwanted and unused liquid coating material from the coated particles. The process is controlled to produce a predominance of droplets of excess liquid coating material of a predetermined size which are smaller than the coated individual mass components.
Uratsuka (Japan Public Patent Disclosure Bulletin No. 2-292324) describes a microcapsule-type urea-type hardening accelerator enclosed within a thermoplastic resin with a softening point of 40.degree. C. to 200.degree. C.
Coated spheres or microcapsules formed by processes such as those referred to above can suffer from the presence of active material on the surface of the coating material, thereby negating the advantages sought by coating or microencapsulation.
It is therefore an object of this invention to form coated spheres of active material, the outer surface of which spheres are free of active material.